WASHINGTON (CNN) -- Three Air Force officers fell asleep while in control of an electronic component that contained old launch codes for nuclear intercontinental ballistic missiles, a violation of procedure, Air Force officials said Thursday.

The Air Force said the launch codes had been deactivated before the incident, but it was still a violation of protocol, prompting an investigation.

It is the fourth incident in the past year involving problems with secure handling of components of America's nuclear weapons.

The incident occurred July 12, during the changing out of components used to facilitate secure communications between an underground missile-control facility and missile silos near Minot Air Force Base in North Dakota, according to Col. Dewey Ford, a spokesman for the Air Force Space Command in Colorado.

One of the parts, a code component, is for storage and processing. It is considered classified by the Air Force.

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A code component was removed from the equipment at the remote missile-control facility and replaced with a new code component. That made the old component inoperable, but an Air Force source said old launch codes were still contained in the part.

Under standard procedure, the four-officer crew of the facility is supposed to keep the component secure until it is returned to the base. Ford said the crew took the component to a building above the facility and locked the component in a lockbox.

Then, three of the four crew members fell asleep.

This violated Air Force procedure, which calls for at least two of the crew members to remain awake while in control of the component. At the time they were asleep, the crew and the component were in a locked building that is guarded by at least one armed airman at all times.

The airmen were asleep for two to three hours, Ford said.

The component was later returned to the Minot base, and the investigations of procedural violations were started by Missile Command, Space Command, the 20th Air Force and the National Security Agency.

The investigation revealed the codes were not compromised, according to the Air Force. The codes had remained secured, and the crew was inside an area protected by Air Force security at all times, the investigation concluded.

The incident, which was first made public by the Project on Government Oversight, was the fourth misstep involving the handling of America's nuclear weapons in the past year.

Last summer, a B-52 flew from Minot AFB to Barksdale AFB in Louisiana carrying six nuclear warheads that were not supposed to leave Minot. The crew of the bomber did not know that it was carrying nuclear missiles.

In March, the Department of Defense discovered that nuclear triggers had been mistakenly sent to Taiwan and left there for 18 months before being returned to the United States.

In May, Minot's 5th Bomb Wing failed a crucial security inspection. The wing was the same unit involved in the B-52 incident.